438 THE EVOLUTION OF THE METAZOA 



even in the burrowing Echinoida where new signs of a bilateral 

 symmetry can be observed re-emerging. The adult forms of 

 the Echinodermata (we state this explicitly), however, can 

 hardly be taken into consideration as possible ancestors of 

 the Chordonia as this is the case with the Chaetognatha. We 

 have emphasized the statement "the adult Echinodermata" 

 because attempts have been made, even by scholars who must 

 be taken seriously, to represent the planktonic and at the 

 same time bilaterally symmetric larvae of the Echinodermata 

 as the initial forms for the evolution of the Chordonia type. 

 The second attempt which was made by the subtypes of 

 the Oligomeria to achieve secondarily a free mobility can be 

 observed in the small group Pterobranchia. The case is interest- 

 ing because these are clearly primarily tubicolous animals. No 

 type of the Eumetazoa could or did ever develop from these 

 forms. Their organization had been strongly and irreversibly 

 reduced. Quite remarkably we can find in the Pterobranchia 

 —in these only and in the Enteropneusta— an open connec- 

 tion between the anterior intestine and the external world. 

 This connection, however, which is supposed to be used by 

 the animal in its respiration, was not invented by the Ptero- 

 branchia; it can also be found in some older types, even 

 among the Ameria (Gastrotricha). This branchiotrema has 

 been the main reason why zoologists have tried to bring the 

 Pterobranchia into a closer connection with the Entero- 

 pneusta which are even better equipped with the gill slits. 

 I believe that zoologists have gone too far with these attempts 

 and that these two groups of the Oligomeria developed 

 independently from each other in a very remote geological 

 past. It seems to me that the Enteropneusta had never really 

 lived as tubicolous animals and that they are therefore the 

 only group of the Oligomeria which possess a primary free 

 mobility that has been preserved by them down to the present 

 day. The possibility cannot be completely excluded, however, 

 that the Pterobranchia— which are now represented by few 

 remaining forms only— had evolved from some burrowing 



